U.S. Pat. No. 4,638,601 describes a system for rotating a circular disc in a first station so as to align the disc in a particular manner so that it can then be transferred linearly without rotation into a second station for mounting on a vacuum chuck for machining. In the first station the disc is rotated so as to determine any eccentricity of the disc relative to the centre about which it is rotated, and this eccentricity is then aligned with the linear transfer path. Having achieved this alignment the centre about which the disc was rotating and the geometric centre of the disc are now aligned with the transfer axis. By so aligning the disc, any error in placement at the first station (which typically can be of the order of a few millimeters) as between the geometric centre and the actual axis of rotation in the first station, is converted into an adjustment in the linear shift along the path from the one station to the other. This enables the disc to be centred accurately at the second station so that the geometric centre of the disc is aligned with the axis of rotation of the vacuum chuck in the second station, thereby enabling accurate grinding of the edge of the disc by a grinding wheel.
There are many situations in which this type of placement is insufficient, since it does not allow discs to be positioned at the second station so as to accommodate all desired forms of rotation and grinding. Thus for example it may be desirable actually to mount the disc eccentrically relative so as to accommodate a damaged region of the periphery of the disc. By locating the disc so that a damaged region is at a greater radius from the geometric centre than is the undamaged remainder of the periphery and rotating the disc about the eccentric axis, a new disc is formed, albeit slightly smaller than the original disc, but in which the damaged region is ground away in the process. The eccentric axis about which the disc is rotated in the second station eventually, on completion of the grinding becomes the geometric centre of the new disc, formed by the grinding process.
Where an alignment method is employed such as disclosed in U.S. Pat. No. 4,638,601, there are only 2 chances in 360 that a damaged peripheral region will lie at one end or the other of the diameter of the original disc which has to be aligned with the transfer path in the prior art method.
It is and object of the invention to provide a workpiece alignment and shifting system which enables a disc to be mounted in the second station for rotation about any chosen axis.
It is to be understood that, the invention is not limited to situations in which the edge of the disc is damaged, but can of course be employed in all situations, provided (as intended), it is possible to mount the disc for rotation about any axis (including the geometric centre of the disc).